1. Field of the Invention
This invention relates to a pressure measuring sheet and a method for measuring pressures using the sheet. More specifically, the invention relates to a pressure-measuring sheet coated with microcapsules having a volume average particle size and a number average capsule wall thickness which are in a specified ratio.
2. Description of the Prior Art
In recent years, there has been a great demand for methods for the dynamic measurement of various planar and linear pressures including, for example, clamping, pressures of bolts in general machinery, the pressurized state of automotive braking systems, various pressures encountered in thermoplastic or thermosetting fabrications such as mold pressures, clamping pressures, contact pressures between resins in molds and mold walls and molding pressures within the molds, abutting planar pressures of springs, clamping planar pressures in joint parts of carburetors, fuel pipes, fuel tanks, etc., in a fuel system, collision pressures between a human body and other objects during an automobile accident, linear and planar pressures of rubber and plastic rolls, checking the effective limits and effective conditions of gasket seals, hydraulic pressures of braking devices, the teeth abutting pressure of gears, contact planar pressures between solids, bolt clamping pressures, abutting pressures of cocks in cock valves, abutting pressures between curved surfaces, abutting pressures during the operation of rolls for rolling steel material, the distribution of pressures between human feet and the surface of a floor and the distribution of pressures between the human body and seats. Despite this, no simple method of measuring such pressures has been developed to date, and available methods are extremely complicated.
Conventional methods for measuring planar and linear pressures include, for example, methods based on strain gauges utilizing the relationship between stress and strain, and methods using load meters or pressure-sensitive paints (strain-sensitive lacquers).
When a strain gauge is used for the measurement of planar and linear pressures, large scale apparatus including an amplifier, a detector, a recorder, etc., must be used. Moreover, techniques for operating these devices and complex calculations involving material strengths are required. This type of method also has the defect that the material undergoing pressure measurement must have surface smoothness. Details of strain gauges are described, for example, in J. Yarnell "Strain Gauge," Corona-sha, etc.
The method using pressure-sensitive paints (strain-sensitive lacquers) has the disadvantages that coating unevenness tends to occur, adhesion of the coating to the material for measurement is unsatisfactory, and operation is complicated.
In the method using a load meter, it is impossible to render the device small in size, and such is unsuitable for measuring planar and linear pressures.
A pressure measuring method using a pressure-sensitive laminate is also known (see U.S. Pat. No. 3,647,504). This method relies on the use of a one-leaf type recording sheet coated with microcapsules of different wall thicknesses containing marking solutions of different colors, and, according to the pressure applied, different microcapsules are ruptured. Since the ruptured capsules contain solutions of different colors, the extent of the pressure applied can be determined from an examination of the colors. Although the extent of pressure can be qualitatively determined by this method, it is difficult to determine an exact value of the pressure applied. From the viewpoint of practical use, the pressure-sensitive laminate causes considerable coloration, and has the defect of soiling the hands and wearing apparel.
In view of this state of the art, we made a series of investigations into methods for measuring pressures of various kinds of sites without requiring large-scale apparatus, a high level of operating technique or complicated calculations. These investigations led to the discovery of an effective method which comprises bringing a recording sheet into contact with an object for pressure measurement, applying pressure thereto to develop a color, and reading the pressure from the change in the optical density or the color of the developed image on the recording sheet, the color development by pressure application being based on the pressure contact of a microcapsular layer containing a color former with a color developer layer.
This method, however, suffers from the defect that when pressure-sensitive sheets or pressure-sensitive copying sheets (as disclosed, for example, in U.S. Pat. Nos. 2,712,507, 2,730,456, 2,730,457, 3,418,250, and 3,425,327) are used as the recording sheet, the precision of pressure measurement within the pressure range of 600 kg/cm.sup.2 or higher is inferior. With a view to remedying this defect, we have furthered our investigations, and found a pressure measuring sheet which exhibits superior measuring precision over pressures within a wide range.